The subject of the present invention is a hermetic compressor intended for circulating gas, such as a scroll-type compressor.
A hermetic compressor, particularly one for providing refrigeration, comprises a pump which raises the pressure of the gas between a low-pressure inlet chamber and a high-pressure pressure chamber or delivery chamber. The pump is operated by an electric motor which has a thermal cut-out device which breaks the electrical supply circuit to the motor when the temperature rises.
Document EP-B-0,480,560 relates to a scroll-type compressor equipped with a thermal cut-out device consisting of a heat-sensitive valve that opens when a predetermined value is reached so as to form a pipe providing communication between the pressure chamber and the inlet chamber of the compressor, when the refrigerant is flowing within the compressor, the motor thermal cut-out device cutting off the electrical supply to this motor.
However, the power supply to the motor is not cut off until the temperature within the entire compressor has increased considerably. This means that the break in power supply to the motor is delayed, and damaging overheat can occur locally within the compressor. The compressor can only be re-started once it has cooled down, and this cooling can be achieved only by thermal conduction through the hermetic chamber.
It should also be noted that placing the inlet chamber and the pressure chamber in communication with each other is not good for the compressor, insofar that it is not possible to be sure that this passage is correctly sealed during normal running conditions, as it is possible that the component that provides the temporary closure may, in time, react less reliably and thus adversely affect the compressor efficiency. Finally, in cases of this kind, the opening of the valve depends not only on the temperature but also on the difference in pressure between the pressure chamber and the inlet chamber, even though the electrical supply to the motor needs to be cut off only if the temperature is too high.
Document U.S. Pat. No. 5,186,613 describes a similar thermal cut-out device, in which a valve situated in a by pass pipe opens when the temperature exceeds a predetermined value. The by pass pipe for connection comes out close to the motor control switch that reacts to temperature, which switch is placed directly in the gas flow when the valve opens. This ensures that the motor is switched off quickly, avoiding a needless increase in temperature of certain regions of the compressor, which means that the compressor can be re-started relatively quickly.
However, in neither of the two devices defined above is the temperature of the pressurized gas assessed directly, insofar that the temperature-sensitive valves are mounted in thermal contact with a component that has significant mass. This means that the valves react to a value which is the result, on the one hand, of the temperature of the gas and, on the other hand, of the temperature of the component or components. Thus the temperature-sensitive valves cannot react to quick increases in temperature of the gas in the pressure chamber. It is somewhat doubtful that the valves will be able to open in an emergency situation, for example when an expansion valve in the external refrigeration circuit is jammed, insofar that the pressure on the inlet side of the compressor will have dropped considerably while the pressure and temperature of the outlet gas will have increased. This will mean that the differential pressure at the valves will be so high that tremendous force will be needed to open them. This in particular is why bi- metallic valves cannot open for high differential pressures.